How much excess carbon dioxide can the ocean hold and how will it affect marine life?

It is 4:30 a.m., far from land. A group of scientists clad in bright yellow foul-weather gear gathers in the open bay of a research ship. They wait in the chill air while the ship’s crew brings their instrument back on board after a 6-mile roundtrip to the ocean floor.

As the sun begins to rise, the scientists carefully remove seawater collected at various depths from the 36 bottles on the rosette-shaped sampler. Meanwhile, the ship steams 30 miles to another part of the ocean, and the process starts all over again—seven days a week, for seven weeks—over 3,600 miles from south to north in the South Atlantic Ocean.

The year was 2005, and the research vessel Ronald H. Brown, operated by the National Oceanic and Atmospheric Administration, was following the same track of a similar seawater-collecting expedition in 1989. The 2005 researchers analyzed thousands of seawater samples for chemical compounds including salt, dissolved inorganic carbon, and chlorofluorocarbons (the now-banned chemicals formerly used in aerosols). The scientists were there to document how the ocean has changed over 16 years—as a result of increasing carbon dioxide levels in the atmosphere and our changing climate.

The oceans have slowed greenhouse warming by absorbing excess heat-trapping carbon dioxide from the atmosphere. But how much we can depend upon the ocean to continue to act as a brake on ever-accumulating CO2 in the future? And will the buildup of CO2 in the ocean change its chemistry, making it more acidic and threatening marine life?

Pouring CO2 into the sky

Carbon dioxide gas traps long-wave radiation (heat) leaving Earth’s surface, thus raising temperatures. Without the warming caused by natural levels of CO2 and water vapor in our atmosphere, the average surface temperature of our planet would be well below freezing.

Atmospheric CO2 levels have varied greatly over Earth’s history, but human activity is significantly altering the global carbon cycle, and not in a good way. Carbon dioxide is rising because of the burning of fossil fuels (oil, coal, natural gas) and because we alter the land through increased farming and the destruction of tropical forests and plants that take up CO2 during photosynthesis.

As a result, CO2 levels are increasing faster than at almost any other time in planetary history. Atmospheric carbon dioxide concentrations are already 30 percent higher than just a couple of centuries ago. Most climate models project that they will reach 2 to 3.5 times pre-industrial levels by the end of this century unless dramatic steps are taken to reduce CO2 emissions.

This higher CO2 will bring warmer temperatures. Climate models predict that global temperatures will increase by 3.5°F to 8°F (1.9o C to 4.4o C) by the year 2100—and even more in the Arctic and Alaska. Beyond the temperature rise, a warmer climate is expected to shift rainfall and drought patterns, which will have even greater consequences for people, wildlife, and ecosystems.

The sea sink

Not all of the excess CO2 we humans emit stays in the atmosphere. The ocean, and to some extent the land, act as large “carbon sinks” that significantly slow the accumulation of atmospheric CO2 and the resulting climate change.

To date, about one-third of all human-generated carbon emissions have dissolved into the ocean. How fast the ocean can remove CO2 from the air depends on both atmospheric CO2 levels and ocean circulation and mixing—in the same way that the sugar dissolving in iced tea depends on how much you put in and how fast you stir. More CO2 in the air leads to more in the ocean; faster circulation increases the volume of water exposed to higher CO2 levels in the air and thus increases uptake by the ocean.

The sleep-deprived scientists collecting water in the South Atlantic were working to learn just how much of the extra CO2 has dissolved in the ocean in the past, and how much is entering right now. This information can help us to better predict how fast atmospheric CO2 levels may rise in the future⎯and what our future climate may look like.

What happens to carbon in the ocean

The hard work of collecting and analyzing thousands of water samples on ships such as the R/V Ronald H. Brown is just the first step toward knowing what happens to carbon in the ocean. Measuring the ocean’s uptake of the added “anthropogenic” CO2 is no easy task, for several reasons.

First, there are large amounts of carbon normally in the ocean—about 50 times the amount in the atmosphere. The increase up to now has been a small, hard-to-distinguish percentage of the total CO2 dissolved in the ocean—about 3 percent in surface waters and only 0.25 percent over the full ocean depth.

So scientists must use advanced analytical techniques to tease out the small anthropogenic CO2 signal from the data. They also measure the ocean’s uptake of other gases that humans put into the atmosphere, such as chlorofluorocarbons (which, unlike carbon dioxide, is chemically non-reactive and remains intact).

The ocean is also a dynamic environment, much like the atmosphere. Currents and storms mix the water, making it hard to track the flow of CO2. The picture is further complicated by plants, animals, and bacteria, which continually take up, release, and transport carbon in the ocean. These create sources and sinks of dissolved carbon that vary across and through the oceans, and during different seasons.

We do know, from several different research approaches, generally where anthropogenic carbon is highest: in the upper 1,000 feet of ocean—near the surface where it enters from the atmosphere—and in cold water that forms near the poles and, being denser, sinks in plumes to intermediate and deep depths.

What’s in store for the ocean?

The question for policy-makers and society is “Will the ocean continue to take up anthropogenic CO2?” Our best evidence is that it will—but less effectively because of interactions between the ocean and the evolving climate.

Several factors come into play. Global warming will inevitably cause seawater temperatures to rise. Warmer water holds less dissolved gas than colder water, so the ocean will not be able to store as much anthropogenic CO2.

A warmer climate will also melt ice and increase rainfall near the poles, adding fresh water to the ocean. Fresh water is more buoyant than saltier water and “floats” on top of it, stratifying the ocean and slowing the mixing and circulation that transports anthropogenic CO2 away from the surface and into reservoirs in the deep ocean. The net effect will be even higher atmospheric CO2 concentrations and a further acceleration of global warming.

Warmer temperatures, weaker circulation, and different stratification of the ocean will have impacts on marine life and ecosystems, which in turn could affect the ocean’s ability to store carbon. How these changes may occur is not clear at this point, however, and may vary from region to region.

A more acidic ocean

The increasing amount of carbon in the ocean will cause another problem for marine life: ocean acidification. The 3-percent increase in dissolved carbon in surface water may seem small, but it is enough to significantly alter the chemistry of seawater and threaten whole groups of marine life.

The reason involves some basic chemistry. When CO2 gas dissolves in seawater, it combines with water molecules (H2O) to form carbonic acid (H2CO3). The acid releases hydrogen ions into the water. The more hydrogen ions in a solution, the more acidic it becomes. Hydrogen ions in ocean surface waters are now 25 percent higher than in the pre-industrial era, with an additional 75-percent increase projected by 2100.

A carbon-containing mineral, calcium carbonate (CaCO3), is a vital component in the ocean, used by many marine creatures to build protective shells and hard structures. Coral reefs, for example, are the accumulation of calcium carbonate skeletons secreted by small coral polyps.

Calcium carbonate shells are also used by several groups of planktonic organisms, microscopic floating plants and animals that are critical and abundant components of the marine food web. The white chalk cliffs of Dover, for example, are made out of empty shells that sank to the bottom of the sea when these organisms died.

The problem is, acidic conditions are corrosive to already formed calcium carbonate, and they also make it harder for organisms to build such hard parts in the first place.

Consequences for marine life

Will corals and shell-forming plankton be able to adapt to a high-CO2 world? We do not know for certain, but preliminary evidence from laboratory and field experiments is not encouraging.

Higher acidity has a negative impact on almost every species examined. In some experiments, you can actually watch the shells of living organisms dissolve away with time.

Especially vulnerable are small marine snails called pteropods and deep-water corals that live in high latitudes, where colder waters have already become more acidic. These species play critical roles in their ecosystems—as food or habitat for other creatures—so the impact of ocean acidification may soon extend to other marine life, including fish and marine mammals.

If you mention “climate change” to people, it often conjures up images of heat waves, melting glaciers, hurricanes, droughts, and monsoon rains—certainly not changes in the ocean, its chemistry, and tiny plankton inhabitants. But we know that future climate change will largely depend on the chemical composition of the atmosphere and the sea—and how vulnerable they are to human perturbation. Understanding how carbon cycles through the Earth system is key to unraveling vital questions about our climate.

Some policy-makers and entrepreneurs have even proposed injecting carbon dioxide into the deep ocean to sequester it from the atmosphere. Ocean carbon-monitoring projects such as the work on the NOAA ship Ronald H. Brown contribute vital data to learn about the ocean’s changing chemistry. Other methods, including experiments that use numerical modeling to form predictions and studies on how ocean acidification affects ocean life, must inform our decisions on how tightly we may want to regulate carbon emissions.

Funding for Scott Doney's and Naomi Levine's work was provided by the Woods Hole Oceanographic Institution Ocean and Climate Change Institute, the National Science Foundation, and the National Oceanic and Atmospheric Administration.

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